The use of polymer films on semiconductor dies as an alpha-particle barrier and as a mechanical insulation layer between the die and the encapsulating material is well known in the art. The upper surface of a semiconductor die is highly vulnerable to mechanical damage during the fabrication process. The sooner a die is covered with a protective coating, such as a layer of polyimide film, the sooner it will be protected against circuit discontinuities and row and column shorts (typically caused by a scratch or foreign particle on the die's upper surface).
The conventional technique for creating a protective polyimide film on the upper surface of a semiconductor die begins by depositing a drop of liquid polyamic acid on the upper surface of each die after it has been attached to one of the die-mounting pads of a leadframe strip and gold wires have been bonded between the connector pads on the die and the corresponding connector pads of the associated leadframe. If the correct amount of liquid has been deposited on the die, it will be dispersed over the die surface, but surface tension will prevent it from spilling over the edge of the die. The liquid polyamic acid is then cured in an oven at temperatures in the 300-degree-Centigrade range, thus polymerizing to form a tough protective polyimide film on the surface of the die.
The process of applying liquid polyamic acid to the die surface is fraught with problems. If too little polyimide is deposited on the die, it will lack sufficient protection against both data-destroying alpha-particle radiation and mechanical damage caused by the encapsulating material rubbing against the surface of the die as the two materials expand and contract at different rates as temperature varies. If too much polyimide, surface tension of the liquid will be insufficient to prevent it from running over the edge of the die, resulting in a film on the surface of the die that is too thin. There are also two problems associated with the high temperatures of the polymerization process. Firstly, the bonding wires quickly become annealed (soft) and have a tendency to become displaced as liquid plastic flows over them during the die encapsulation process. Such displacement may be severe enough to cause the wires to short out against the edge of the die. Secondly, it has been discovered that the heating of the die during the polymerization process permanently degrades performance of the chip.
Recognizing the advantage of protecting semiconductor dies early in the fabrication process (prior to wafer sawing, die attachment and wire bonding processes), New Long Corporation of Japan has developed a fully-automated screen printer for applying polyamic acid to all the dies on a wafer simultaneously. The printer, known as the New Long LS-15TVF, employs a dual camera system for aligning a printing screen with the fiduciary marks on a wafer. Although the process definitely represents a quantum advance in the art, there are a number of problems associated with screen printing. Firstly, as with the conventional technique, the liquid requires curing in an oven, which may lead to decreased reliability of the die. Secondly, the viscosity and rheology of the liquid must be maintained in a narrow range for the process to work properly. And thirdly, the maximum thickness of a polyimide layer applied to a wafer by a screen-print process followed by oven curing is approximately 50 microns. For certain applications, polyimide layers of double that thickness are required, making the screen-print process unusable.